feat(home-manager): Add platform compatibility guards to cross-platform roles

Add lib.optionals pkgs.stdenv.isLinux guards to roles that contain
Linux-only packages or services to prevent build failures on Darwin:

- communication: Guard Electron apps (element-desktop, fluffychat,
  nextcloud-talk-desktop) that don't build on Darwin due to electron
  build-from-source limitations
- kdeconnect: Guard entire config block since services.kdeconnect
  requires D-Bus and systemd (Linux-only)
- sync: Guard syncthingtray package (requires Linux system tray)
- email: Guard systemd.user.services/timers (Darwin uses launchd)
- desktop: Guard Linux-only packages, services, and KDE-specific
  configurations including gnome-keyring, systemd services, and
  XDG mime associations

Implements bead: nixos-configs-tcu

home/roles/kdeconnect/default.nix

@@ -4,13 +4,15 @@ with lib;
 
 let
   cfg = config.home.roles.kdeconnect;
+  isLinux = pkgs.stdenv.isLinux;
 in
 {
   options.home.roles.kdeconnect = {
     enable = mkEnableOption "Enable KDE Connect for device integration";
   };
 
-  config = mkIf cfg.enable {
+  # KDE Connect services are Linux-only (requires D-Bus and systemd)
+  config = mkIf (cfg.enable && isLinux) {
     services.kdeconnect = {
       enable = true;
       indicator = true;